A photosensitive, heterobifunctional, cleavable cross-linking reagent (XL) was used to isolate CTL surface proteins ("contact proteins") that make contact with target cells (TCs) and possibly participate in delivering a "lethal hit" in transient CTL-TC conjugates. The TCs were coated diffusely on their surface by XL.Con A and then mixed with 125I-labeled CTLs. The resulting con-jugates were exposed to UV light to induce cross-linking and then extracted with detergents. Antibodies to Con A were added, immunoprecipitating soluble complexes that consisted of "contact proteins" covalently bound to XL.Con A which, in turn, was found noncovalently by the anti-Con A antibodies. Reductive cleavage of S-S bond in XL yielded 125I-labeled CTL proteins, which were analyzed by SDS PAGE. The labeled, immunoprecipitated proteins with molecular weight (in kilodaltons) of approximately 210, 190, 180, 160, 120, 105, 80, 47, 38, and 32 seem to be "contact proteins," i.e., they were not immunoprecipitated if the CTL TC conjugates were not UV-irradiated, if XL was omitted, or if conjugate formation was impaired. The approach described here may provide a new way to isolate CTL surface proteins of functional significance for TC recognition and lysis. (CS)